The likelihood of a river bursting its banks and flooding is determined by factors in the surrounding landscape, such as steepness of the river valley, the amount of vegetation and the prevailing rock-type. The short-term impact of floods can be catastrophic, but they can have positive long-term effects as well.

Causes of flooding

A flood occurs when a river bursts its banks and the water spills onto the floodplain. Flooding tends to be caused by heavy rain: the faster the rainwater reaches the river channel [river channel: The part of the river that holds the water. ], the more likely it is to flood. The nature of the landscape around a river will influence how quickly rainwater reaches the channel.

The following factors may encourage flooding:

Helicopter carrying British aid across flood water, Mozambique

A steep-sided channel - a river channel surrounded by steep slopes causes fast surface run-offsurface run-off: The water that runs over the surface of the land when the soil is unable to absorb it..

A lack of vegetation or woodland - trees and plants intercept precipitation (ie they catch or drink water). If there is little vegetation in the drainage basin then surface run-off will be high.

A drainage basin [drainage basin: The area of land that is drained by a river and all its tributaries. ], consisting of mainly impermeableimpermeable: not allowing water to pass through rock - this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.

A drainage basin in an urban area - these consist largely of impermeable concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of run-off.

Flood management techniques include river engineering, afforestation [afforestation: The deliberate planting of trees on otherwise bare land. Afforestation can create new habitats for wildlife, stabilise soils and prevent surface run-off. ] and planning controls to restrict urban development on floodplains.

BBC links

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.